Hi @Ksavshs . You can try configuring the Archer as Dynamic IP (instead of PPPoE) and connect its WAN port to a LAN port of main router. The Archer will get a dynamic ip address from main router.
Or, define it as Static IP and set ip address, etc.
You can make the Archer wifi names and passwords same as main router. On the 2.4GHz band, use different channel on each router. Channels 1, 6, 11 have least interference.
Firmware upgrade isn't going to make a difference in this mode.